High data rate packet based systems, such as those of the Institute of Electrical and Electronics Engineers (IEEE) 802.11 family of standards (commonly known as WiFi), are becoming more widespread as the number of WiFi enabled devices being used continues to grow. Packet based communication generally operates under the assumption that the received signal is stationary over a packet time. Each received packet may consist of a training period followed by a data period. The a-priori known training period may be used for functions such as automatic gain control (AGC) and channel estimation, which may then be used for data detection in the data portion of the packet. Since the receiver does not typically have a-priori knowledge of the expected receive power, an AGC system is typically employed so that the incoming signal may be properly scaled before further processing.
AGC system performance generally depends on the ability to accurately estimate receive signal power. The initial gain setting may be set to a high level to ensure that low power signals are detected at the receiver. In a typical receiver configuration, a variable gain amplifier (VGA) may reside before an analog-to-digital converter (ADC). Due to this configuration, there is a potential for heavy saturation at the output of the ADC, where receive signal power estimation is normally performed. Heavy saturation may cause power to be underestimated when using estimators that do not account for clipping. This may lead to a slow AGC convergence time. Fast AGC convergence time is desirable in general, but is especially important for high rate packet based systems, because these systems allow for only a limited number of updates in the early stages of a training period of a received packet. This is due to the desire to fix the VGA gain relatively early in the preamble portion of the packet. Furthermore, optimal setting of the VGA gain requires statistics sufficient to estimate the received signal distribution. Various features of current AGC systems prevent fast AGC convergence, especially in high data-rate packet based systems. Thus, methods and apparatus to compute signal and noise measurements that take ADC saturations into account are desired. In addition, methods and apparatus for appropriately setting the gain level at the VGA, quickly and accurately, are also desired.